What happens if you don't have enough credits to buy that same ball?
To continue without delay, you'd have to use the Starter balls. In a MP game, you'd be forced to do so because the game advances without much delay. Alas, the fun with your friends will - hopefully - cover that handicap,
In a single game, you have plenty options because you may let the game idling without action, or you may freeze (save) it for a long time, limited only by the given deadline if it's a tournament.
Caveat: Idling may not work on mobile(?), and practice rounds cannot be saved.
You may then go to the Pro Shop, buy credits, then buy new - identical! - balls. You may even turn to "Earn free credits", enter surveys or run videos to gain credits.
You may also have a friend gift you balls (identical!).
After filling up, return to your round.